The present invention pertains generally to interferometers and more particularly to interferometers for determining plasma density.
FIG. 1 illustrates a typical prior art coupled cavity interferometer. The advantage of this device is that it uses a single external reflector which is aligned with a laser source through a contained plasma. As such, the device is easily aligned and simply adapted to various sytems in which measurements of plasma density are required.
Notwithstanding these advantages, the coupled cavity interferometer cannot track multi-fringe phase excursions unambiguously, has limited differential sensitivity, is affected by refractive bending of the scene beam and requires external calibration.
FIG. 2 schematically illustrates a quadrature interferometer which utilizes polarization optics to obtain signals, i.e., sin .phi. and cos .phi., which unambiguously identify multi-fringe phase excursions. Moreover, the quadrature interferometer has excellent and uniform differential sensitivity, is relatively immune to small refractive bending in the scene beam and is automatically calibrated. Unlike the coupled cavity interferometer, however, the quadrature interferometer requires alignment of numerous optical elements as shown in FIG. 2, which in many cases, cannot be accomplished due to space limitations. In addition, the polarizing optics are expensive and time consuming to align to achieve the desired results.